Migrating site to C5
Permalink/og.php
which posts to:
/og2.php
both files have a require_once to:
/inc/og_lib.php
which contains the php functions(not mvc) to make the feature work. There is database interaction in the feature.
Should I just create single pages and use a separate db? If so, do I need to add og.php, og2.php & og_lib.php to the dashboard?
Is it better to somehow use the c5 db? What is the best way to make this work in c5?
Please provide me with some specifics as I am still trying to sort out when I need a block/single page, or other. I am new to object oriented programming and mvc, usually just use old school functions, so any help is appreciated.
Thanks for help!

If you don't want 2 databases, you could add all of the tables from the old database into the concrete5 database and change the code in og_lib.php to reference the concrete5 database instead.
For a heavily-customized form I don't see any reason to do any more integration than that. The only exception being if you needed this form to be a "block" that users could add to any page in your site -- but note that this will require a *LOT* of work, so make sure it's really worth it (I'm guessing probably not).
Hope that helps.
-Jordan
DIR_BASE.'/libraries/og_lib.php';
(note that the DIR_BASE constant is defined by concrete5, so it will work if you're using it from your single_page controller, but not if it's just a free-standing script file).
If you're including them server-side (so you need a URL as opposed to a file path), use DIR_REL instead of DIR_BASE:
DIR_REL . '/js/myfile.js'; DIR_REL . '/images/myimage.jpg';
although note that if you're including the image from a CSS file, the DIR_REL thing won't work -- you'd need to use a relative path from wherever the css file, which should probably go in "css/", then it would be url(../images/myimage.jpg);